This invention relates to metallic coatings and, more particularly, to metallic coatings including aluminum applied to internal and external surfaces of an article.
An important effort in the evolution of gas turbine engines has been the development of high temperature operating coatings to protect the surface of certain engine components from environmental attack and degradation. Generally, such higher temperature operating components are of a metal based on an element selected from Fe, Co, Ni and Ti. The more advanced designs of such components as turbine blades included such cavities as channels and small holes within the blade and communicating with various surface portions of the blade to allow a cooling fluid such as air to pass through and reduce the temperature of such a component.
Although there have been reported a wide variety of coatings and methods for applying coatings to the outer surface of such components, the ability of such methods to coat the internal surfaces of small holes, channels and other internal cavities is greatly restricted. For example, the diffusion aluminiding process of the general type described in U.S. Pat. No. 3,667,985 -- Levine et al, issued June 6, 1972, used for external coatings, is limited in throwing power, i.e., in its ability to coat very far into holes with high length/diameter ratios. Similarly, electoplating techniques cannot plate inside narrow chambers or holes because electric fields are excluded. Physical vapor deposition and thermal spraying are essentially line-of-sight processes that cannot deposit on the hole surfaces.